Re: HELP With Step-Down Transformer and Converter

On 19 Sep 2005 13:29:52 -0700, "gene" <winapps2@xxxxxxxxxxxxxxx> wrote:

>Dear Phantom;
>
>I can't thank you enough for your help.
>Don't worry, I will be careful. I will stand in another room holding
>the switch, wear goggles, leather gloves, and all the necessary
>precautions. I know it is dangerous to handle such high voltage.

Actually, it's high *current*, not high voltage you're dealing with in the DC setup, and
in the AC setup the high voltage (240 volts) is only on the primary side of the
transformer. But really high currents pose a danger of a different sort. You won't get
electrocuted or even shocked by the high current circuits you propose to create. The
danger is molten metal splattering around.

>I have my camcorder setup to record evrything that happens so I really
>don't need to be close.
>
>Your assumption is correct. I only want to get a high current in a wire
>for its
>magnetic effects, and not to drive a load, and YES, I would not mind
>spending some money on it.
>
>In the Dc-Dc setup using gel cells, do I need to have an interruptor?

Your are thinking that you need an "interruptor" to get the DC to provide "transformer
action" so it can be stepped down in voltage and up in current. With the lead acid
battery, this is not needed because a transformer is not needed. This is why I recommend
the gel cells rather than a DC-DC converter. It's much simpler.

But, remember in my previous post, I said:

"You must have some disconnect device
in series with the battery (near one terminal of the battery), such as
a DC rated circuit breaker:
http://www.solarseller.com/dc_circuit_breakers_12_volts_to_125_volts.htm
or fuse."

That's a type of "interrupter", but it's not to get "transformer action". It's more
commonly called a "disconnect". The idea is that if anything goes wrong while the
experiment is running and some of the wiring becomes red hot (this has happened to me as a
teenager. We all learn some of these lessons the hard way), when you panic and try to
undo the bolts connecting the wiring to the battery, you discover that red hot wires burn
you. You need a disconnect so that all you have to do is flip the switch and everything
is turned off. The disconnect should be the first thing in the circuit connected to one
of the battery terminals where no other wire can bypass around it. What I am recommending
is that you use the big DC rated circuit breakers as switches to turn the current on and
off, and since they are breakers too, they will automatically disconnect if the current
gets too high. If you chose to use a fuse for the current limiting function, you must
still have a switch there for the safety disconnect. That's the advantage of breakers;
they are switches too. (Such high current fuses are expensive, by the way.)

In your high current experiments the danger of molten metal comes from making
connections when the circuit is energized. When you touch two pieces of metal which will
carry 3000 (or even 750) amps when the connection is made, the sparking at the point of
contact can splatter molten metal. You should turn off the breaker or switch while making
the connections to the ammeter and whatever else is in the high current circuit. This is
just common safe practice when working with electricity. Don't work on energized
circuits. Hook everything up the way you want it, *then* turn on the switch or breaker.
If you made a mistake and too much current passes, the breaker will open and avoid serious
damage. Then you have to troubleshoot your setup and figure out what you did wrong.

For your AC setup, the house breaker in the 240 volt circuit will provide the safety
disconnect in the event of a short circuit. If you build the transformer I described, you
should mount it in an appropriate electrical enclosure (box) with a switch on the primary
(240 volt) side so you can turn it on and off while you're working on the high current
side without having to run to the house breaker box. You don't want to have to provide a
switch on the 3000 amp secondary side; it would be very big and expensive and it's not
necessary. Just turn off the 240 volts to the primary of the transformer when you're
working on the high current side. The breakers in the house service entrance are often
not designed to be used as switches for many on-off cycles anyway, so the extra switch
takes the wear and tear off the house breaker.

>I understand that if I use AC, I do not need an interruptor since AC is
>already turning ON and OFF by itself, but in a DC setup, shouldn't I
>have an interruptor to induce a high current on the secondary?

The DC setup I described doesn't need an interruptor. It doesn't have a transformer and
so there isn't a secondary; there's just the battery. The lead acid cells can put out a
very large current on their own. Have you ever shorted a car battery accidentally when
connecting jumper cables? When you start your car, the battery is supplying hundreds of
amps to the starter motor. Lead acid batteries are dangerous because of the high currents
they can supply. Most modern cars have a plastic sheath that covers the terminal(s) of
the battery, because if you are working on your car and accidentally drop a wrench across
the terminals of the battery without that sheath, you get a most impressive fireworks
display!
>
>To Answer your question on the soldering gun:
>How do you know you didn't get high current? How did you try to measure
>it?
>>>> I used a tester to measure the tip of the soldering gun.
>
>If you got heat, then you got high current.
> I maybe using the wrong equipment to measure current... like the AC sensitive ampmeter you suggested.
> I was just wondering, if the soldering gun produces a high current at it's tip,

The high current is only in the tip; it doesn't get into the thing you're soldering.

> would it not short circuit the CHIP that one is soldering or would it not electricute the user holding the lead?

The soldering gun has essentially the circuit I described for your AC setup. It's just
a transformer with a single shorted turn of very heavy conductor for a secondary. If you
remove the actual tip that does the soldering and measure the voltage at the bolt-on
connections, it's probably quite low. My own Weller soldering gun has .35 volts open
circuit at the bolt-ons, and .24 volts when the tip is in place. That voltage isn't
enough to hurt anything. The current in the tip of a 100 watt gun must therefore be about
100/.24, or about 400 amps. But the reason that can't hurt you is that the 400 amps can
only flow in the copper tip, not in your body nor in the thing you're soldering. It takes
much more *voltage* to cause that much current in a human body (thousands or volts,
although it doesn't take 400 amps to electrocute a person so a much lower voltage than
thousands can electrocute, but .35 volts is completely harmless through skin). Only a
virtual short circuit, such as provided by the heavy copper tip on the soldering gun can
cause that much current when only .24 volts is available to "push" the current.

>
>What is a rare earth magnet? Where can I buy one?

A rare earth magnet is a magnet made from mixture of substances which include what are
called the "rare earths". The elements samarium and neodymium are the two most common
rare earth elements used in modern magnets. These magnets are *very* strong, and the big
ones can hurt you by pinching you really hard when a couple of them smack together on your
fingers. You can buy them at:
http://www.wondermagnets.com/cgi-bin/edatcat/WMSstore.pl?user_action=list&category=Magnets_and_Magnetism%3BPermanent_Magnets%3BNeodymium
Get a couple at least of number 0013 and several number 0016 and whatever else strikes
your fancy. They are fun to play with! Don't let them get close to your watch, your TV
or your computer monitor or any floppy disks; and especially don't let them get close to
your credit cards. They will erase the magnetic strip on the card; don't even hold a
magnet in your hand and drop your hands down to your waist near a pocket with your wallet.

>Lastly, Mike suggested that I should just buy a portable harbor freight
>220V spot welder... Does this portable welder use DC as source? Is
>MIke correct in saying that I would achieve and be able to measure the
>same high current induction?

The designation 220V means that it gets its power from the 220 volt AC coming into your
house. I wouldn't know what it does after that without seeing a spec ***, but Murphy's
law says that it won't do what you want.

>
>My deepest gratitude to your efforts in helping me.
>
>Gene

.

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